Torque at different temperatures

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BadDog
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Re: Torque at different temperatures

Post by BadDog » Thu Dec 21, 2017 12:55 pm

Whether torque spec require lube or not depends on the source of the spec. For example, most torque specs given in the older automotive service manuals are dry torque, with exceptions specifically called out. It's always a very controversial topic covering all sorts of details, like what to do when an "in frame" rebuild makes it basically impossible to provide for a dry torque (correction factors). More recent equipment/manuals often specify torque specs lubricated, torque to yield, and measured stretch. Others specify with such narrow tolerance calibrated drivers are required.
Russ
Master Floor Sweeper

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Rich_Carlstedt
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Re: Torque at different temperatures

Post by Rich_Carlstedt » Sun Dec 31, 2017 1:41 pm

Russ, exactly right !
The thing to know is what purpose is the torque requirement ?
In our die shop work, all our torques were for "Maximum Tensile Strength"
That means you want the highest strength possible for the given fastener used aand this is determined by the screw/bolt maker.
This has several implications !!!!
What is the thread pitch ( and size) ?
What is the fastener material ?
For example, if a tractor manufacturer uses a grade 5 screw and you use a Grade 3 , you will never reach a "Max" strength spec !
( "IF" that is what it was )
conversely if he uses a grade 3 and you use a grade 8 , you will have lots of room in over torquing.
If you substitute a fine thread for the same size ( say a 1/2-20 instead of a 1/2-13) , you will head for disaster if you use the same torque !
But wait, what if they did not give the "Max Strength" level of torque , but instead only looked at the sealing load between the component parts ? All this is important because of the following "LAW" for fasteners.
Whenever you use the Maximum torque for a fastener ( THAT also was determined by the Screw Manufacturer !) , you should-- NO must-- toss that fastener whenever it is removed. The reason is that when you reach maximum torque set by the screw/bolt manufacturer. the screw is distended ( stretched) when placed under operating load and can never return to it's former size. Subsequent use of that screw prevents it from ever reaching it's designed strength level as it reached the yield point during it's life .

SO in a nut shell, if a screw has a 100 pound torque rating and a 85 pound "Lubriacated torque rating by the screw maker, it will tell you that is what the fastener is capable of. So when the machine manufacturer says the parts gets a 75 pound torque, the screw is good to go for multiple times. The machine maker determines if they want it lubricated based on the screw use and location.

I mentioned the above, because head bolts on automotive engines always come up when guys talk about torque.
I have no axe to grind,only to point out that the auto maker determines how much force he wants (PSI) on the head gasket and how many bolts ( Diameter and TPI) he has available and that "may" determine the torque spec.
To be knowledgeable about it you need to also know what the screw maker specs call out as well------ so you can see the "error" tolerated.

Rich

pete
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Re: Torque at different temperatures

Post by pete » Sun Dec 31, 2017 2:50 pm

Rich's point about bolt failure when reusing them at close to there maximum torque jogged my memory. Most of the large earth moving equipment I've run is CAT and items like the special square headed bolts used on the replacable cutting edges are expensive since 30 or more large bolts and nuts might be used. But they are as expected excellent quality. Good enough they even have the CAT logo forged into the head. I've worked for a few company's who tried to save money by reusing them when changing to new cutting edges. There usualy installed with a 3/4 drive or larger impact gun on the larger machines and tightened as much as possible. With the reused bolts it was a given you'd see at least one broken bolt within the first 2 hrs and I've broken as many as 15 in a 12 hr shift. It was also extremely rare to break one if new bolts were used each time. For anything of higher value like internal engine components that might see even mild racing or safety items I'd sure think long and hard before reusing any highly stressed bolts.

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liveaboard
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Re: Torque at different temperatures

Post by liveaboard » Sun Dec 31, 2017 4:38 pm

Back when I was working in an auto repair garage in the '80s, I was about to torque down some head bolts with a torque wrench.
"What are you doing??" my [GM trained] boss demanded, "Don't waste time, use an impact wrench! Do you think they use a torque wrench at the factory??"
Well I hadn't been to the factory, but he had.

While working for him, I put down many [iron] heads with an impact wrench; but when I work on my own vehicles, I use a torque wrench.
Yeah, the bolts stretch, you can feel it.
When the stretching starts, the torque required to turn the bolt falls away.
Then, if you keep going, there's a 'bang'.
Oops.
Anyone got a helicoil?

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wlw-19958
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Re: Torque at different temperatures

Post by wlw-19958 » Sun Dec 31, 2017 7:30 pm

Hi There,
liveaboard wrote:Back when I was working in an auto repair garage in the '80s, I was about to torque down some head bolts with a torque wrench.
"What are you doing??" my [GM trained] boss demanded, "Don't waste time, use an impact wrench! Do you think they use a torque wrench at the factory??"

The impact wrenches used on the assembly line have a preset
torque to tighten the bolts to a specified torque. I worked as
a GM mechanic in the 1970's and early 1980's and I always used
a torque wrench when tightening head bolts.

Good Luck!
-Blue Chips-
Webb

spro
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Re: Torque at different temperatures

Post by spro » Mon Jan 01, 2018 5:58 am

Well there it is. There is a certain "gift of skill" which we acquire as much as a cello player. We can't state it but it is there. WE Know torques and when a fastener is going to break if more tension is applied. You can only "feel" it with a wrench.
Same stuff happens. The bolt doesn't break and set it aside for maybe something else. Dang !

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